bytemaster0

All,

I seem to be encountering an issue on a 2015 E250BT4 (a bit over 11,000 miles) that has only crept up in recent days where it's been much hotter on average. I cannot decide whether this is a bug or a feature:

When the car has been sitting in the hot sun (the *air* doesn't need to be that hot - 85°F and up is enough to repeat this) for about an hour or two, I notice that after starting, the transmission seems to want to delay upshifting. Under normal circumstances, when I get on the highway, usually I'm in 7th by the time I reach 55 MPH. When it's been sitting in the hot sun, 4th, 5th, and 6th seem to "stick" by one gear (meaning it'll stay in 6th when it should have upshifted to 7th, and so on), though I can of course manually override by the paddle shifters. After a period that is directly proportional to the amount of time spent in the sun, the car will all of a sudden behave normally. This can be as soon as 10-15 seconds, or as long as 5 minutes. The hotter the outside air temperature, and the longer it sits in the sun, the longer it takes to shift "normally."

To add more information, my wife's GLK250BT (also 4MATIC), does NOT seem to exhibit this behavior. Perhaps only once or twice, and it was *brutally* hot outside when it did. Note that both cars feature the OM651 4-cylinder twin-turbodiesel engine. The GLK, however, is notably geared higher (by this I mean that it tends to run at higher RPMs in the same speed range, I assume to make up for greater aerodynamic drag). Also, if the car has been in a garage or covered space, the issue is generally not noticeable. And, if the car is parked in the garage overnight, at say, 75°, I don't notice the issue at all.

I had a discussion with my dealer, and my wonderful service manager gave it her full attention. She spoke promptly to the shop foreman, who seemed to think that this may not be normal. However:

I summoned my inner mechanical engineer to try to figure out this issue:

These engines are forced-induction engines; they use two turbocharges to force more air into the cylinders. So, by simple thermodynamics, the cooler the intake air temperature, the more air can be compressed by the turbochargers due to greater air density, and the more power the engine can produce. If, by contrast, the intake air temperature is hotter, and thus less dense, then the engine will not be able to generate as much power. Furthermore, these engines are fitted with intercoolers whose job it is to cool the intake air that has is undergoing compression. (I'm not sure where the intercooler sits - before, between, or after the turbos, though I expect either between or after.) Could it be, that to preserve the same pedal feel, the transmission delays upshifts while the intake cooling system is getting to a cooler temperature? Again, this is extremely repeatable, and directly proportional to how long the car has been sitting in direct sunlight. The hotter the air, and the stronger the sun, the longer the transmission seems to wait before acting normally. I know that the cars do this in *cold* weather to heat up the catalyst and bring the engine to a more efficient operating temperature, but I'm surprised to see the same effect in hot weather, albeit for a very different reason.

I suspect I rarely see this behavior on the GLK is because it is already working at higher RPMs for the same speed range. In fact, at 70 MPH the GLK250BT sits at around 1800-1900 RPM, which is right around the peak of the torque band. The E250BT4 generally is also in 7th gear at 70 MPH, but usually at around 1500-1600 RPM.

So, am I crazy, or am I sane? Does my conclusion make any sense? I firmly believe that MB engineers would have chosen to do this to preserve the "pedal feel" even if the intake air temperature is too hot to provide optimal compression, and thus power, especially in a car that likes to run at lower RPMs and upshift earlier under normal circumstances.

Does anyone have the same experience? Or, should I have this looked into more by the dealer to see if something's not working as it should? I'd appreciate your thoughts. Thanks!

Trancebolt

The gear ratios are the same. Final drive is too. Could be rim and tire size . Also the e250 4matic is heavier too. ~3-400lbs and yeah the height diff will affect it. The drag coefficient is likr .27 vs .32 or higher your right. E is faster as well

As far as your theory on heat and turbochargers youre right. Thre are multiple cooling devices as well. Plus just because the oil is up to temp doesnt mean the metal is. So initial 5 mins can exhubit those protection symptons for the trany.

they do say 75 or maybe higher % of all engine damage and wear is done while its not at operating temps.

my 350 diesel with one larger turbo has similar signs too. You can always upshift while in D (auto) and it will force upshift. I dont agree that it prefers low revs. It has the most efficient output low revving but even at the weakest point in the power band it has as much torque as a e350. (255@redline vs270).

bytemaster0

Thanks for your input, Trancebolt, much appreciated! Keep in mind that I'm generally talking about a hot car - engine was just running, AC was running, and it's been left in the sun. If I leave it parked for 5 minutes in the hot sun, it will want to delay upshifts. If I leave it longer in the hot sun, it delays them even longer. However, on a cool weather day? Normal. Even when it was in the teens during the winter, and the car had been run within an hour or two, it would shift completely normally.

Huh! That's interesting to know - makes total sense that engine and transmission layout might be identical. Must be different wheel size - thought, to be honest, the GLK has *larger* wheel/tire combinations than the E - the GLK has 28.3" diameter wheel/tire combinations, while the E has a 25.7" diameter wheel/tire combination. So, by that logic, shouldn't the E run at higher RPMs due to smaller wheel size? It runs at much lower RPMs than the GLK, in this case. I think the E is several hundred RPM lower at the same speed. Would that not be indicative of a different gearing at the transmission? Would be interesting to know.

You are right. Cars with the OM642 engine, like yours, tend to rev higher. My dad has had a W211 with that engine, and several MLs with variants of the 642. I've also had a used ML320CDI 4MATIC with the same engine - it likes revs a bit higher, as you've stated. However, by a somewhat noticeable contrast, the E250BT4 with the 4-cylinder OM651 does tend to want to shift up earlier. Gets great mileage by doing so.

Anyway, thanks again for your feedback - if anyone with an E250BT is experiencing the same issue, please let me know, I'd really appreciate it.

bytemaster0

So I think I've found the key component that's causing this phenomenon - the AC. If I shut off the AC, the transmission immediately begins to operate "normally." As soon as I turn it on, and it goes to full cooling, the transmission will actually downshift by itself until the temperatures start to normalize. Very interesting.

thefisch

doesn't the alternator have a higher output at higher rpm. could that be the connection?

Disclaimer: I am not a mechanic.